Influence of Backside Energy Leakages from Hadronic Calorimeters on Fluctuation Measures in Relativistic Heavy-Ion Collisions

Andrey Seryakov

arxiv: 1907.05703 · v1 · pith:STYOFPDCnew · submitted 2019-07-12 · ⚛️ physics.ins-det · hep-ex· nucl-ex

Influence of Backside Energy Leakages from Hadronic Calorimeters on Fluctuation Measures in Relativistic Heavy-Ion Collisions

Andrey Seryakov This is my paper

Pith reviewed 2026-05-24 22:08 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-exnucl-ex

keywords heavy-ion collisionscentrality selectionmultiplicity fluctuationsspectator energy lossMonte Carlo generatorsNA61/SHINE

0 comments

The pith

Inefficiency in selecting central collisions by forward spectators dramatically alters multiplicity scaled variance in light systems but barely affects heavy ones.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper investigates how energy leakages from the backside of hadronic calorimeters influence fluctuation measures in heavy-ion collisions by simulating spectator loss. It applies SHIELD and EPOS Monte Carlo generators plus the wounded-nucleon model to introduce a probability of losing forward spectators and their energy. The results indicate that light colliding systems such as Be or Li suffer large distortions in multiplicity scaled variance from even small selection inefficiencies. Heavy systems such as Ar + Sc remain largely unaffected. This distinction arises because spectator number correlates more tightly with particle multiplicity in smaller systems.

Core claim

Introducing a probability to lose forward spectators and spectator energy in SHIELD and EPOS simulations combined with the wounded-nucleon model demonstrates that light systems experience dramatic changes in multiplicity scaled variance from minor centrality selection inefficiencies, whereas heavy systems show little sensitivity to the same effect.

What carries the argument

Monte Carlo modeling of forward spectator energy loss and its correlation with produced-particle multiplicity via the wounded-nucleon model.

If this is right

Light-system data require near-perfect centrality efficiency or explicit corrections to yield reliable fluctuation measures.
Heavy-system fluctuation results remain usable even with modest spectator losses.
System-size scans must treat centrality selection efficiency as a size-dependent systematic uncertainty.
Multiplicity variance in the lightest beams may need reanalysis if calorimeter acceptance for spectators is incomplete.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Experiments using forward calorimetry for centrality in small systems should quantify spectator loss directly from data rather than relying solely on generators.
The same modeling approach could be applied to other fixed-target or collider setups to predict bias in fluctuation observables.
One testable extension is to vary the loss probability continuously and map the resulting variance distortion as a function of system size.

Load-bearing premise

The SHIELD and EPOS generators together with the wounded-nucleon model correctly reproduce the spectator energy distribution and the correlation between spectator number and produced-particle multiplicity in real collisions.

What would settle it

Direct comparison of multiplicity scaled variance measured in real Be+Be or Li+Li data against the same quantity extracted from the simulations while varying the spectator loss probability would confirm or refute the modeled size-dependent impact.

Figures

Figures reproduced from arXiv: 1907.05703 by Andrey Seryakov.

**Figure 1.** Figure 1: A scheme of the NA61/SHINE experiment [10] and a front view photo of the Projectile Hadronic Calorimeter. PSD is used for triggering, centrality and event plane determination. Signals from 60 scintillators in each module are grouped by six, therefore, the design of PSD allows collecting information from ten independent areas along the beam axis inside the calorimeter. This makes it possible to study depend… view at source ↗

**Figure 2.** Figure 2: A sketch of how a collision volume depended quantity behaves with the increase of a centrality calorimeter length. Two MC datasets were generated with the GEANT4 [11] PSD simulation for studying the energy leakage influence on different systems: 100,000 events of 150A GeV/c 7Li + 9Be SHIELD MC [12] and 40,000 events of 150A GeV/c 40Ar + 45Sc EPOS 1.99 MC [13]. 7Li was chosen instead of experimentally used … view at source ↗

**Figure 3.** Figure 3: Comparison of different measures behavior versus the centrality calorimeter length for Li + Be (blue dots) and Ar + Sc (red dots) 150A GeV/c collisions. The first two plots present an average number of negatively charged hadrons, the second two show the negative charged hadrons scaled variance, next four show two strongly-intensive quantities ∆[Pt,h] and Σ[Pt,h] [14] and the last two show a ratio of the av… view at source ↗

**Figure 4.** Figure 4: Detected forward spectators distributions for Li + Be, Cl + Ca and Pb + Pb in WNM with a probability to loss a nucleon of 0% and 10%. 0 20 40 60 80 100 probability to lose a spectator 15 20 25 30 35 <N>, Li+Be 17 GeV 10% 0 20 40 60 80 100 probability to lose a spectator 60 80 100 120 140 160 180 200 220 <N>, Cl+Ca 17 GeV 10% 0 20 40 60 80 100 probability to lose a spectator 600 700 800 900 1000 1100 1200 <… view at source ↗

**Figure 5.** Figure 5: Multiplicity and scaled variance versus the probability to lose a forward spectator in WNM with a centrality selection based on the number of detected forward spectators [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗

**Figure 6.** Figure 6: Ratios of multiplicity and scaled variance to values with a zero probability to lose a forward spectator for Li + Be (blue), Cl + Ca (red) and Pb + Pb (black) collisions. WNM with a centrality selection based on the number of detected forward spectators. Unexpectedly, this simple model reproduced two important features: higher sensitivity of light systems for small energy loss and lower sensitivity for lar… view at source ↗

**Figure 7.** Figure 7: A ratio of deposited energy by a 150 GeV/c proton in the sections from 8 to 20 to the whole long calorimeter PSD (11.2 nuclear int. lengths) in the GEANT4 simulation. The whole calorimeter model has 20 sections. This distribution shows the fraction of a proton energy leak from a calorimeter, which has 3.9 nucl. int. lengths or seven sections in case of PSD. The 10% of the most central events were selected … view at source ↗

**Figure 8.** Figure 8: Energy distributions of forward spectators with realistic energy leakage from the calorimeter in WNM [PITH_FULL_IMAGE:figures/full_fig_p007_8.png] view at source ↗

read the original abstract

The phase diagram of the strongly interacting matter is the main research subject for different current and future experiments in high-energy physics. System size and energy scan programs aim to find a possible critical point. One of such programs was accomplished by the fixed-target NA61/SHINE experiment in 2018. It includes six beam energies and six colliding systems: p + p, Be + Be, Ar + Sc, Xe + La, Pb + Pb and p + Pb. In this study, we discuss how the efficiency of centrality selection by forward spectators influences multiplicity and fluctuation measures and how this influence depends on the size of colliding systems. We use SHIELD and EPOS Monte-Carlo (MC) generators along with the wounded nucleon model, introduce a probability to lose a forward spectator and spectator energy loss. We show that for light colliding systems such as Be or Li even a small inefficiency in centrality selection has a dramatic impact on multiplicity scaled variance. Conversely, heavy systems such as Ar + Sc are much less prone to the effect.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper shows via MC that spectator leakage biases scaled variance far more in light systems than heavy ones, but the size of the effect is tied to unvalidated model assumptions about spectator-multiplicity correlations.

read the letter

The main result is that even modest losses of forward spectators can inflate multiplicity scaled variance dramatically for light systems such as Be+Be or Li, while the same inefficiency has only a mild effect in heavier systems such as Ar+Sc. The authors demonstrate this by running SHIELD and EPOS generators inside the wounded-nucleon framework and introducing a tunable probability for spectator energy loss or escape. They apply the setup to the exact beam energies and systems of the 2018 NA61/SHINE scan. That targeted quantification for the light-ion part of the program is the concrete new piece; earlier centrality studies did not isolate this particular leakage channel at this level of system-size comparison. The work is useful in flagging a detector-related systematic that could matter for fluctuation analyses in fixed-target light-ion runs. The qualitative trend with system size follows from simple counting: when the total spectator number is small, losing one or two changes the selected sample more than when the total is large. The soft spot is exactly the one the stress-test note flags. The reported magnitude of the bias is produced inside the chosen generators; the paper gives no indication that the joint distribution of spectator energy and produced multiplicity was tuned or cross-checked against real NA61 data or alternative models. If that anti-correlation is weaker or stronger in nature than in SHIELD/EPOS, the size of the effect shifts. Because the full text was not supplied here, I cannot check whether later sections add such validation or error bands. This is the sort of note that NA61 analysts and people running similar fluctuation measurements should read so they can decide whether to add a dedicated correction or run their own generator tests. It is not a foundational paper, but it raises a practical issue worth referee attention. I would send it to peer review with referees who know both the generators and the NA61 forward calorimeter.

Referee Report

2 major / 2 minor

Summary. The paper studies the effect of inefficiencies in centrality determination arising from energy leakages of forward spectators in hadronic calorimeters on multiplicity fluctuation measures. Using SHIELD and EPOS Monte Carlo generators together with the wounded-nucleon model, the authors introduce a probability for spectator loss and energy leakage, and report that light systems (Be+Be, Li+Li) exhibit a dramatic inflation of multiplicity scaled variance even for small inefficiencies, while heavier systems (Ar+Sc) remain comparatively robust.

Significance. If the modeling assumptions hold, the result provides a concrete warning about a previously under-appreciated systematic for fluctuation analyses in small collision systems, directly relevant to the NA61/SHINE system-size scan. The quantitative demonstration of system-size dependence is a useful contribution for experimental design and data interpretation.

major comments (2)

[Methods / Simulation setup] The central quantitative claims (dramatic impact for Be/Li, robustness for Ar/Sc) rest entirely on the joint distribution of spectator energy and produced-particle multiplicity generated by SHIELD/EPOS plus the wounded-nucleon model. No comparison of this correlation to NA61 data or to alternative generators is shown, which is load-bearing for the reported effect sizes.
[Results] The abstract and results state that even small spectator loss produces large changes in scaled variance for light systems, yet the manuscript provides no sensitivity study varying the generator parameters or the wounded-nucleon assumptions that control the width of the spectator-energy distribution at fixed centrality.

minor comments (2)

[Introduction] Notation for scaled variance and the precise definition of the inefficiency parameter should be introduced earlier and used consistently.
[Figures] Figure captions should explicitly state the beam energies and system combinations shown, and whether statistical uncertainties are displayed.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments. We address the major comments below.

read point-by-point responses

Referee: [Methods / Simulation setup] The central quantitative claims (dramatic impact for Be/Li, robustness for Ar/Sc) rest entirely on the joint distribution of spectator energy and produced-particle multiplicity generated by SHIELD/EPOS plus the wounded-nucleon model. No comparison of this correlation to NA61 data or to alternative generators is shown, which is load-bearing for the reported effect sizes.

Authors: Our study is a Monte Carlo investigation using SHIELD and EPOS generators together with the wounded-nucleon model to demonstrate how spectator detection inefficiencies can affect fluctuation measures. The effect sizes are model-dependent by nature, and we have chosen these widely used tools for their relevance to NA61/SHINE analyses. No direct comparison to data is included because the paper focuses on the potential bias rather than fitting to existing measurements. The results are shown to be consistent between the two generators. We will revise the text to better emphasize the model-based nature of the quantitative results. revision: partial
Referee: [Results] The abstract and results state that even small spectator loss produces large changes in scaled variance for light systems, yet the manuscript provides no sensitivity study varying the generator parameters or the wounded-nucleon assumptions that control the width of the spectator-energy distribution at fixed centrality.

Authors: We recognize the value of a sensitivity analysis. The use of two different generators provides a basic check on the robustness to modeling differences. The wounded-nucleon model is applied in its standard implementation. To address the comment, we will include an additional discussion or figure exploring the dependence on the spectator energy width in a revised version of the manuscript. revision: yes

Circularity Check

0 steps flagged

No significant circularity; simulation study applies external generators without self-referential reduction

full rationale

The paper performs a Monte-Carlo modeling exercise using the external SHIELD and EPOS generators plus the wounded-nucleon model to quantify how spectator energy leakage affects scaled variance in different system sizes. No equations, fitted parameters, or predictions are defined in terms of the target observables; the reported differences between light (Be+Be) and heavy (Ar+Sc) systems emerge directly from the chosen external event generators rather than from any internal redefinition or self-citation chain. The central claim is therefore a conditional result inside the adopted MC framework and does not reduce to its own inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The study rests on the domain assumption that the chosen Monte-Carlo generators faithfully model spectator energy leakage and its correlation with multiplicity; no free parameters are explicitly fitted in the abstract, and no new entities are introduced.

axioms (1)

domain assumption SHIELD and EPOS generators plus the wounded-nucleon model correctly capture the relation between forward spectator energy and produced-particle multiplicity
Invoked when the authors introduce a loss probability and recompute fluctuation measures

pith-pipeline@v0.9.0 · 5717 in / 1261 out tokens · 22873 ms · 2026-05-24T22:08:29.945009+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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On Normalization of Strongly Intensive Quantities

Gazdzicki, M.; Gorenstein, M.I.; Mackowiak-Pawlowska, M. On Normalization of Strongly Intensive Quantities. Phys. Rev. C 2013, 88, 024907. [CrossRef]

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